fix: use data-cy selector for sidebar backdrop in e2e tests

The aria-hidden="true" selector matched 27 elements (icons, etc.)
causing cy.click() to fail. Add data-cy="sidebar-backdrop" to the
overlay div and target that specifically in tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/cypress/e2e/mobile_sidebar.cy.ts

@@ -102,7 +102,7 @@ describe("/dashboard - mobile sidebar", () => {
 
     cy.get("[data-sidebar]").should("have.attr", "data-sidebar", "open");
     cy.get("aside").should("be.visible");
-    cy.get('[aria-hidden="true"]').should("exist");
+    cy.get('[data-cy="sidebar-backdrop"]').should("exist");
   });
 
   it("mobile: click backdrop closes sidebar", () => {
@@ -114,7 +114,7 @@ describe("/dashboard - mobile sidebar", () => {
     cy.get("[data-sidebar]").should("have.attr", "data-sidebar", "open");
 
     // Click the backdrop overlay
-    cy.get('[aria-hidden="true"]').click({ force: true });
+    cy.get('[data-cy="sidebar-backdrop"]').click({ force: true });
 
     cy.get("[data-sidebar]").should("have.attr", "data-sidebar", "closed");
     cy.get("aside").should("not.be.visible");

frontend/src/components/templates/DashboardShell.tsx

@@ -136,6 +136,7 @@ export function DashboardShell({ children }: { children: ReactNode }) {
           className="fixed inset-0 z-40 bg-black/30 md:hidden"
           onClick={toggleSidebar}
           aria-hidden="true"
+          data-cy="sidebar-backdrop"
         />
       ) : null}